This invention pertains to masks for x-ray or deep x-ray lithography, particularly to masks formed on graphite membranes or other graphite substrates.
The fabrication of x-ray lithography masks is currently a lengthy and expensive process. It is the object of this invention to provide reliable x-ray or deep x-ray masks that may be produced more quickly and less expensively than has previously been possible.
Masks for deep X-Ray lithography are typically made from fragile, micrometer-thin membranes, typically formed of Si or Ti, and absorber structures 10-15 xcexcm in height to provide sufficient contrast. The fabrication of these masks is a multi-step, lengthy, and expensive process, typically using an intermediate x-ray mask that in turn was directly patterned with an electron beam. The intermediate mask is used to copy the pattern into a thick polymethylmethacrylate (xe2x80x9cPMMAxe2x80x9d) resist using soft x-rays. A metal (usually gold) is electroplated into the thick, patterned resist layer to form an absorber pattern of sufficient thickness to achieve adequate contrast when harder x-rays are used.
Masks for ultra-deep X-ray lithography are usually made in a simpler and less expensive way, for example using thick Si wafers as a substrate. Optical lithography is used to transfer a pattern into a thick UV resist to generate a resist pattern up to 50 xcexcm thick, and the pattern is then filled with 30-35 xcexcm of Au. However, the smallest structures achieved are typically xe2x88x9210 xcexcm, and these masks are generally suited only for hard x-ray sources.
The three main criteria for a high quality x-ray exposure are: (1) The top-to-bottom dose ratio for exposure through the thickness of the resist should be held below approximately 5 (an approximate figure whose value depends on the material used for the resist and its thicknessxe2x80x94it is typically smaller with the thicker resist layers used in ultra-deep X-ray lithography). (2) The dose on the bottom of the resist should be at least about 3000 J/cm3 (again, depending on the material of the resistxe2x80x94the figure given is for PMMA). (3) The contrast between the exposed regions and the unexposed regions should be such that the maximum dose underneath the absorber is about 100-150 J/cm3 (a figure dependent on the material of the resist, the resist thickness, and the development procedures used).
As a consequence, to pattern thicker resists a xe2x80x9charderxe2x80x9d x-ray spectrum is typically needed to effectively expose the bottom of the resist. (Harder x-rays can be used to expose thick resists relatively uniformly, while softer x-rays can overexpose the top of a thick resist before the bottom is properly exposed). A thicker absorber pattern is needed to provide sufficient contrast with harder X-rays.
Fabrication of a mask suitable for patterning 1000 xcexcm of PMMA can take up to six months, and generally costs several thousand dollars.
Several methods have previously been used to fabricate x-ray lithography masks. The methods differ depending on the substrates used, and the processes used to generate the absorber pattern.
A general description of procedures that have typically been used previously to generate X-ray masks is the following:
(1) Electron beam lithography or an optical pattern generator is used to transfer a pattern from a drawing onto an intermediate x-ray mask or onto an optical (typically chromium) mask.
(2) In an intermediate x-ray mask, the resist is approximately 3-4 xcexcm thick, and the Au absorber pattern is approximately 2-2.5 xcexcm thick. This pattern is transferred using xe2x80x9csoftxe2x80x9d x-rays (a few keV photon energy) into a 30-100 xcexcm thick PMMA resist applied onto either a membrane or a substrate.
(3) In case of a chromium mask, optical lithography is used to pattern a thick optical resist onto either a membrane or a substrate, typically using UV light.
(4) In either case, following exposure a wet-chemical development process is used to remove either the exposed regions (for a positive resist) or the unexposed regions (for a negative resist).
(5) The recesses produced in the previous step are then filled with an absorber by electroplating, typically with Au. A typical height for the Au absorber is about ⅔ of the resist height.
(6) The resist is removed, and the mask is mounted onto a carrier for use in x-ray lithography.
(7) In the case of a membrane-based mask, the substrate must be mounted onto a frame or carrier prior to patterning. Such mounting requires special fixtures and extreme care in handling. In the case of a substrate-based mask, the frame or carrier may be mounted after patterning the mask, allowing the use of standard fixtures (typically vacuum chucks) and only moderate attention in handling.
For prototype development work, the time and cost required to manufacture an x-ray mask using this procedure can be excessive.
A graphite-based substrate for an x-ray or deep x-ray mask would be relatively inexpensive, would have reasonably good x-ray transmission, and could be mechanically sound. Such substrates could be used in otherwise standard or ultra-deep x-ray lithography processes.
Graphite substrates have not generally been used in x-ray masks, primarily because they have been considered too xe2x80x9cdirtyxe2x80x9d for use in the xe2x80x9cclean roomxe2x80x9d environment used for x-ray lithographic processes. Graphite surfaces tend to be rough and porous. Silicon substrates and membranes are more familiar to most users.
J. Gxc3x6ttert et al., xe2x80x9cLithographic Fabrication of Graphite-based X-ray Masks,xe2x80x9d paper presented at 42nd Electron Ion and Photon Beam Technology and Nanofabrication Conference, Chicago, Ill. (1998) discloses a process in which graphite sheets are attached to a silicon wafer using PMMA as a bonding layer, and a metal layer consisting of a thin copper layer on top of a chromium layer is sputtered onto the graphite. Then a PMMA resist layer is spin-coated onto the sample. The sample is then exposed with x-rays through an intermediate x-ray mask, and developed to remove exposed resist and to etch the thin copper layer. Then the sample is electroplated with gold, the remaining PMMA is dissolved away, and the remaining copper is etched away.
X-ray lithography masks formed of graphite have been made by micromilling techniques, and lithographically through the use of an intermediate X-ray mask. See P. Coane et al., xe2x80x9cFabrication of composite X-ray masks by micromilling,xe2x80x9d SPIE Proceedings, vol. 2880, pp. 130-141 (1996); C. Friedrich et al., xe2x80x9cMicromilling development and applications for microfabrication,xe2x80x9d Microelectronic Engineering, vol. 35, pp. 367-372 (1997); C. Friedrich et al., xe2x80x9cMetrology and quantification of micromilled x-ray masks and exposures,xe2x80x9d SPIE Proceedings, vol. 3048, pp. 193-197 (1997); and P. Coane et al., xe2x80x9cGraphite-based x-ray masks for deep and ultradeep x-ray lithography,xe2x80x9d J. Vac. Sci. Technol. B., vol. 16, pp. 3618-3624 (1998).
P. Coane et al., xe2x80x9cFabrication of HARM structures by deep-X-ray lithography using graphite mask technology,xe2x80x9d Microsystem Technologies, vol. 6, pp. 94-98 (2000) (paper presented at Third International Workshop on High Aspect Ratio Microstructure Technology (June 1999)) discloses the manufacture of graphite masks in which a single graphite wafer accommodates, on opposite faces, both an intermediate mask and a working mask.
It is an object of this invention to provide a method to fabricate deep X-ray lithography and ultra-deep X-ray lithography masks reliably and inexpensively, facilitating the cost-effective production of high aspect ratio microstructures using either xe2x80x9chardxe2x80x9d or soft X-ray sources.
We have discovered an improved method for producing x-ray or deep x-ray masks on graphite membranes (or thicker graphite substrates) inexpensively (under $1000) and rapidly (within about one day once an optical mask is available). The novel method eliminates the need for an intermediate x-ray mask, instead using a less expensive intermediate UV lithography step (either exposure through a UV mask, or UV direct writing). The absorber structures are electroplated directly onto the graphite. The capability to economically produce x-ray or deep x-ray masks is expected to greatly enhance the commercial appeal of x-ray lithography in processes such as LIGA. (xe2x80x9cLIGAxe2x80x9d is a German acronym for lithography, electroplating, and molding.)
The x-ray mask produced by this process comprises a graphite substrate that supports an absorber such as gold or gold-on-nickel. The thickness of the absorber structures can be varied as needed to supply sufficient contrast for the particular application in question, even for radiation sources with characteristic photon energies up to 40 keV or more. The thickness of the absorber structures may be from a few micrometers to 20 micrometers or more.
A layer of a deep ultraviolet resist is applied directly onto a graphite substrate, typically by spin-coating a layer 30-50 xcexcm thick, but potentially up to 1000 xcexcm thick, depending on the application. The ultraviolet resist is then patterned with a UV mask using a UV radiation source. After developing the exposed resist, gold-on-nickel or other absorber structures are electroplated directly into the resulting gaps in the resist-covered graphite. Once the remaining resist is removed, attaching a graphite membrane (e.g., 125 to 250 xcexcm thick) to a frame completes the mask. If a thicker graphite substrate is used (e.g., 250 to 1000 xcexcm thick), a separate frame support may not be needed.
The novel combination of a graphite-based substrate with deep UV resist offers a number of advantages over competing mask fabrication technologies. The use of a graphite substrate makes it possible to fabricate masks that can be used for both deep and ultra-deep X-ray lithography (depending upon the thickness of the substrate selected), leading to a reliable and standardized fabrication procedure. The combination of graphite and deep UV resists makes it possible to fabricate X-ray masks quickly, without the need for a time-consuming, costly, intermediate mask step.
Typical mask fabrication steps in the novel process, using graphite wafers and deep UV resist are as follows:
(1) Surface preparation of graphite. We have found that commercially available graphite substrates can have different surface qualities and treatments. It can therefore be useful to prepare the graphite surface prior to use by fly-cutting, polishing, or both to improve surface uniformity. In addition, it is usually preferred to seal the surface, for example by depositing a thin metal layer through otherwise conventional means (e.g., sputtering copper or chromium), in order to improve process stability.
(2) Application of the resist. A deep UV resist, for example SU-8 or SJR, is applied to the graphite, for example by spin coating a 10-60 xcexcm thick layer, depending upon the application.
The resist is soft-baked for a suitable time and temperature, depending upon the particular resist and the size of the structures. The resist is exposed with UV in the desired pattern, and then developed. A metal absorber, typically gold, is electroplated into the pattern. The height of the absorber structures should be less than the thickness of the UV resist. The UV resist is removed, and the graphite-gold x-ray mask is mounted in a frame (if necessary or desirable for the intended application).
The x-ray mask produced by this process comprises a graphite substrate that supports an absorber such as gold. The thickness of the absorber structures can be varied from a few micrometers to 30 micrometers or more, to supply the contrast needed for a particular application, even for radiation sources with characteristic photon energies up to 40 keV or more.
As used in the specification and claims, a xe2x80x9cdeepxe2x80x9d ultraviolet resist is an ultraviolet resist that may be effectively patterned to a depth of at least about 10 xcexcm by exposure to ultraviolet light and development, preferably to a depth of at least about 30 xcexcm. A preferred deep ultraviolet resist is the negative resist EPON(copyright) resin SU-8. SU-8 is a multifunctional glycidyl ether derivative of bisphenol-A novolac, available for example from Shell Chemical. SU-8 is a deep ultraviolet resist fabricated from a novolac resin, a negatively acting chemically amplified epoxy, a photosensitive initiator, and gamma butyro lactone as a solvent. SU-8 provides a high epoxy functionality. The novolac resin and epoxy, along with the initiator, form a three-dimensional, insoluble polymer upon exposure to ultraviolet light. The solvent serves to allow the resist to be applied, and is removed by pre-baking.
Another preferred deep ultraviolet resist is one of the SJR family of positive resists, such as SJR 3138, SJR 3440J, SJR 5440, SJR 3740, and SJR 5740. Particularly preferred is SJR 5740, available from Shipley Company (Marlborough, Mass.).
Examples of deep ultraviolet resists other than SU-8 and SJR 5740 include AZ PN114, a novolac-based negative resist available from Shipley Company (Marlborough, Mass.) or Hoechst; and CAMP6, a positive deep UV resist. CAMP6 is a polyhydroxystyrene-sulfone copolymer-based resist available from OCG Microelectronics Materials (West Paterson, N.J.) or Bell Laboratories (Murray Hill, N.J.). Other polyhydroxystyrene-sulfone copolymer-based resists may also be used in this invention.
The structure of the functional portion of SU-8 is 